1. Field of the Invention
The present invention relates to a noise canceler and a receiving apparatus using the same and relates to a technology for improving quality of a signal reproduced by a receiving apparatus.
2. Description of the Related Art
FIG. 7 shows a configuration of an FM receiving apparatus 1. The FM receiving apparatus 1 comprises an antenna 10, a front end (FE) circuit 11 for performing tuning and high frequency amplification of a received signal and converting the received signal to an intermediate frequency signal, an AGC (AGC: Automatic Gain Control) circuit 12 for controlling an amplification gain in the FE circuit 11 depending on field strength of the intermediate frequency signal, an IF stage signal processing circuit 13 comprising an amplifying circuit for amplifying the intermediate frequency signal and a limiter circuit, an FM detecting circuit 14 for demodulating the intermediate frequency signal, an S-meter 15 for outputting an alternating current (S-AC) and a direct current (S-DC) corresponding to the field strength of the intermediate frequency signal based on the intermediate frequency signal output from the AGC circuit 12, a multipath noise detecting unit 16 for outputting a multipath noise detection signal that is a signal indicating presence or absence of the multipath noise based on the alternating current (S-AC), a composite stage signal processing circuit 17 to which the demodulated signal, the multipath noise detection signal, and the direct current (S-DC) are input and which attenuates the demodulated signal during a period in which the multipath noise is included in the signal, a stereo demodulating circuit 18 for generating a main channel signal (L+R) and a sub-channel signal (L−R) by performing matrix processing of the signal output from the composite stage signal processing circuit 17, a pulse noise detecting unit 19 for outputting a pulse noise detection signal that is a signal indicating presence of a pulse noise such as an ignition noise and a mirror noise contained in the intermediate frequency signal output from the AGC circuit 12, a noise canceler 20′ for removing the pulse noise contained in the main channel signal (L+R) and the sub-channel signal (L−R) output from the stereo demodulating circuit 18 based on the pulse noise detection signal, and an audio stage signal processing circuit 21 for generating an L signal and an R signal based on the main channel signal (L+R) and the sub-channel signal (L−R).
The direct current (S-DC) output from the S-meter 15 and the multipath noise detection signal output from the multipath noise detecting unit 16 are input to the audio stage signal processing circuit 21. The audio stage signal processing circuit 21 has an SP (SP: SeParation) processing unit 211 for performing separation processing of changing a degree of separation between the main channel signal and the sub-channel signal, and an HC (HC: High Cut) processing unit 212 for performing high-cut processing of improving an S/N ratio by removing a high frequency component, based on these signals.
FIG. 8 shows a specific configuration of the noise canceler 20′ and the audio stage signal processing circuit 21. The noise canceler 20′ comprises a low-pass filter (LPF) 202 with a cut-off frequency of 15 kHz provided for the purpose of cutting a high-frequency noise contained in the main channel signal (L+R) and the sub-channel signal (L−R), a de-emphasis processing unit 203, and a noise canceling processing unit 201′ for removing the noise by performing interpolation processing such as a linear interpolation for a period to be determined based on the pulse noise detection signal (hereinafter, interpolation period).
The audio stage signal processing circuit 21 comprises a high-pass filter (HPF) 213 for cutting a direct current component contained in the main channel signal (L+R) and the sub-channel signal (L−R) and a multiplexer (MPX) 214 for generating an L signal and an R signal by the main channel signal (L+R) and the sub-channel signal (L−R).
Length of the interpolation period during which the interpolation processing is performed by the noise canceling processing unit 201′ is stored as a fixed vale in the noise canceler 20′. The noise canceling processing unit 201′, upon detecting the presence of the pulse noise by the pulse noise detection signal, sets the length of the period corresponding to the fixed value from the time of the detection of the pulse noise as the interpolation period and performs the interpolation processing of the signal during thus set interpolation period. (see Japanese Patent Application Laid-Open Publication Nos. 1990-283129, 2001-36422, and 2005-277565)
Since the low-pass filter 202 causes a pulse width of the pulse noise mixed in the main channel signal (L+R) and the sub-channel signal (L−R) to be extended, the fixed value must be set at the length sufficient to cover such extended pulse width of the pulse noise. For this reason, the interpolation period is lengthened and quality of reproduced signal of the FM receiving apparatus is unnecessarily deteriorated.
Description will be made more specifically. For example, when the main channel signal (L+R) and the sub-channel signal (L−R) shown in FIG. 9A are input from the stereo demodulating circuit 18, the signal output from the low-pass filter 202 will have a waveform shown in FIG. 9B. The signal output from the de-emphasis processing unit 203 will have the waveform shown in FIG. 9C and the signal output from the noise canceling processing unit 201′ will have the waveform shown in FIG. 9D. The signal output from the high-pass filter 213 will have the waveform shown in FIG. 9E and the signal output from the multiplexer 214 will have the waveform shown in FIG. 9F.
When a comparison is made between the waveform of the original signal input from the stereo demodulating circuit 18 shown in FIG. 9A and the waveform of the signal output from the noise canceling processing unit 201′ shown in FIG. 9D, the pulse noise has the pulse width on the order of 0.02×10−3 in FIG. 9A, while the interpolation period has the width of 0.2×10−3 or over in FIG. 9D and the interpolation period is of the order of 10 times as long as the pulse width of the original pulse noise. For this reason, as shown in FIG. 9F, the waveform of the signal output from the multiplexer 214 is considerably distorted as compared with the waveform of the original signal.